Hitherto, various thermosetting epoxy resin compositions comprising an epoxy resin, a low-reactive hardening agent and a hardening promoter have been known. Such thermosetting epoxy resin compositions are required to have excellent storage stability in order that the hardening reaction of the compositions does not proceed during storage of the compositions. Additionally, such compositions are required to have excellent thermosetting reactivity in order that the compositions may rapidly harden under heat. However, in such thermosetting epoxy resin compositions, storage stability and thermosetting reactivity properties are, in theory, somewhat contradictory to each other. In general, therefore, it is difficult for such compositions to satisfy both properties of high storage stability and high thermosetting reactivity. In fact, no composition is known which can adequately satisfy both of these properties.
For instance, in order to improve storage stability, formation of complexes with a reaction promoter or formation of inclusion compounds with a cyclodextrin has been proposed. In these cases, however, there is a drawback in that the hardening reactivity of the resin composition is undesirably lowered. On the other hand, incorporation of microcapsules containing a reaction promoter into the resin composition has also been proposed (JP-B-43-17654--the term "JP-B" as used herein means an "examined Japanese patent publication"). In this case, however, there is a problem in that the cost of manufacturing the composition is unacceptably high for practical purposes. Additionally, there is another problem in that the microcapsules incorporated are easily broken. Incorporation of a hardening promoter which is stable to heat and which is latent-hardening, for example, a combination comprising tetraphenyl phosphonium tetraphenyl borate and triphenylphosphine (JP-A-61-113614--the term "JP-A" as used herein means an "unexamined published Japanese patent application") or an addition compound obtained by reacting a tertiary amine compound having a hydroxyl or mercapto group and a polyfunctional epoxy compound (JP-A-60-4524), into the resin composition has also been proposed. However, the illustrated techniques also have some drawbacks, i.e., the composition having good storage stability is unsatisfactory with respect to the hardening reactivity thereof, while the composition having satisfactory hardening reactivity has an unsatisfactory storage stability.
JP-A-63-161018 discloses a method of obtaining a thermosetting epoxy resin composition having both excellent storage stability and excellent hardening reactivity, in which a low-reactive hardening agent, a hardening promoter and a zeolite are blended while the hardening agent and the hardening promoter are melted, the resulting blend is cooled and solidified, the resulting solid is powdered into a fine powder, and the powder is incorporated into an epoxy resin composition. However, since the powder which is used as a hardening agent in this method has a water-absorbing property caused by the presence of the zeolite, the storage stability property is still insufficient. Accordingly, where the powder is left under conditions of a high humidity for a long period of time, the powder absorbs moisture. When such a wet powder is combined with an epoxy resin and the resulting epoxy resin composition is used as a powdery coating composition, it has been found that the coated film formed by the composition often causes some outward appearance abnormalities. Further, the hardening reactivity of the powdery coating composition is not always uniform.